1. Field of the Invention
The present invention relates generally to integrated circuits and, more specifically, the present invention relates to the modulation of light using integrated circuits.
2. Description of the Related Art
Within the integrated circuit industry there is a continuing effort to increase integrated circuit speed as well as device density. One challenge that integrated circuit designers face with increasing circuit speeds and device densities is the increasingly significant propagation delays of circuit inputs and outputs due to the capacitive loading associated with off chip circuit connections. At slower clock speeds, the capacitive loading on integrated circuit lines is generally not a significant factor. However, as newer integrated circuit design clock speeds continue to climb towards the gigahertz range and beyond, it is evident that one of the major bottlenecks for future integrated circuits, such as for example but not limited to microprocessors, off chip caches, controllers, etc., will be the input/output bandwidth and/or round trip delay between and within chips.
Prior art attempts to address the capacitive loading problems associated with increased integrated circuit speeds and device densities have resulted in the use of larger and more powerful integrated circuit input/output drivers on the chip. Undesirable consequences of utilizing larger input/output drivers include the facts that the larger input/output drivers generally consume more power, create large di/dt noise, which requires low inductance packaging and large amount of on-die decoupling capacitance to provide a means of noise suppression, dissipate more heat and occupy more of valuable area on the integrated circuit die than smaller integrated circuit input/output drivers.
Other prior art attempts to overcome traditional integrated circuit interconnection limitations have included the use of optical interconnections. The prior art attempts at optical interconnections between integrated circuits have generally involved or have been based on two typical approaches.
One approach has been based on either using gallium arsenide (GaAs) laser diodes and modulating or switching the diodes electrically or by using GaAs built modulators that amplitude modulate a laser beam passing through the integrated circuit. The modulation is generally based on electroabsorption through strained multi-layer grown molecular beam epitaxy (MBE) films in GaAs integrated circuits. As can be appreciated to those skilled in the art, it is difficult and therefore impractical to integrate or combine III-V based technology, which includes GaAs, with standard silicon based complementary metal oxide semiconductor (CMOS) technology.
The second typical prior art approach is based on using silicon based optical waveguides. These waveguides are generally built using Silicon-on-Insulator (SOI) based processing techniques. Prior art SOI based modulators utilize silicon waveguide structures to switch light passing through the optical waveguide. The switching mechanism however utilizes injection of carriers into the waveguide rather like in a bipolar based transistor. One consequence of this is slow speed, for example up to several hundred megahertz, and very high power consumption, for example 10 mW or more for a single switch. In order to increase the modulation depth, one often tries to obtain a large interaction volume between the injected charge and the light beam. This is generally accomplished by making very long waveguides, for example on order of thousands of microns, thereby increasing the interaction length through which the light beam travels. As can be appreciated to those skilled in the art, actual incorporation of SOI waveguides into existing multi-layer standard CMOS based processing however is not straight forward. Hence, utilization of these waveguide structures becomes quite impractical when used for high speed input/output in large transistor count microprocessors.
Accordingly, what is desired is a method and an apparatus for optically modulating light using integrated circuit elements with voltage or current signals. Such a method and apparatus should enable signals to be extracted from integrated circuits for uses including, for example, optical input/output, silicon debug testing, etc. In order to reduce the impact on present day CMOS processes, such a method and apparatus should be compatible with present day CMOS integrated circuit processing technology. It is also desired that one should be able to implement such a method and apparatus without significant change to existing process technology. In addition, such a method and apparatus should also be compatible with present day flip chip package technology.